Ion implantation device and a method of semiconductor manufacturing by the implantation of boron hydride cluster ions

ABSTRACT

An ion implantation device and a method of manufacturing a semiconductor device is described, wherein ionized boron hydride molecular clusters are implanted to form P-type transistor structures. The molecular cluster ions have the chemical form B n H x +and B n H x −, where 10&lt;n&lt;100 and 0≦x≦n+4. The use of such boron hydride clusters results in a dramatic increase in wafer throughput, as well as improved device yields through the reduction of wafer charging. A method of manufacturing a semiconductor device is further described, comprising the steps of: providing a supply of molecules containing a plurality of dopant atoms into an ionization chamber, ionizing said molecules into dopant cluster ions, extracting and accelerating the dopant cluster ions with an electric field, selecting the desired cluster ions by mass analysis, modifying the final implant energy of the cluster ion through post-analysis ion optics, and implanting the dopant cluster ions into a semiconductor substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Provisional PatentApplication No. 60/463,965, filed on Apr. 18, 2003, entitled “An IonImplantation Device and a Method of Semiconductor Manufacturing by theImplantation of Boron Hydride Cluster Ions,” and U.S. patent applicationSer. No. 10/183,768, filed on Jun. 26, 2002, entitled “Electron ImpactIon Source.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of semiconductor manufacturingin which P-type doping is accomplished by the implantation of ion beamsformed from ionized boron hydride molecules, said ions being of the formB_(n)H_(x) ⁺ and B_(n)H_(x) ⁻ where 10≦n≦100 and 0≦x≦n+4.

2. Description of the Prior Art

The Ion Implantation Process

The fabrication of semiconductor devices involves, in part, theintroduction of impurities into the semiconductor substrate to formdoped regions. The impurity elements are selected to bond appropriatelywith the semiconductor material so as to create electrical carriers,thus altering the electrical conductivity of the semiconductor material.The electrical carriers can either be electrons (generated by N-typedopants) or holes (generated by P-type dopants). The concentration ofdopant impurities so introduced determines the electrical conductivityof the resultant region. Many such N- and P-type impurity regions mustbe created to form transistor structures, isolation structures and othersuch electronic structures, which function collectively as asemiconductor device.

The conventional method of introducing dopants into a semiconductorsubstrate is by ion implantation. In ion implantation, a feed materialcontaining the desired element is introduced into an ion source andenergy is introduced to ionize the feed material, creating ions whichcontain the dopant element (for example, in silicon the elements ⁷⁵As,³¹p, and ¹²¹Sb are donors or N-type dopants, while ¹¹B and ¹¹⁵In areacceptors or P-type dopants). An accelerating electric field is providedto extract and accelerate the typically positively-charged ions, thuscreating an ion beam (in certain cases, negatively-charged ions may beused instead). Then, mass analysis is used to select the species to beimplanted, as is known in the art, and the mass-analyzed ion beam maysubsequently pass through ion optics which alter its final velocity orchange its spatial distribution prior to being directed into asemiconductor substrate or workpiece. The accelerated ions possess awell-defined kinetic energy which allows the ions to penetrate thetarget to a well-defined, predetermined depth at each energy value. Boththe energy and mass of the ions determine their depth of penetrationinto the target, with higher energy and/or lower mass ions allowingdeeper penetration into the target due to their greater velocity. Theion implantation system is constructed to carefully control the criticalvariables in the implantation process, such as the ion energy, ion mass,ion beam current (electrical charge per unit time), and ion dose at thetarget (total number of ions per unit area that penetrate into thetarget). Further, beam angular divergence (the variation in the anglesat which the ions strike the substrate) and beam spatial uniformity andextent must also be controlled in order to preserve semiconductor deviceyields.

A key process of semiconductor manufacturing is the creation of P-Njunctions within the semiconductor substrate. This requires theformation of adjacent regions of P-type and N-type doping. An importantexample of the formation of such a junction is the implantation ofP-type dopant into a semiconductor region already containing a uniformdistribution of N-type dopant. In this case, an important parameter isthe junction depth, which is defined as the depth from the semiconductorsurface at which the P-type and N-type dopants have equalconcentrations. This junction depth is a function of the implanteddopant mass, energy and dose.

An important aspect of modern semiconductor technology is the continuousevolution to smaller and faster devices. This process is called scaling.Scaling is driven by continuous advances in lithographic processmethods, allowing the definition of smaller and smaller features in thesemiconductor substrate which contains the integrated circuits. Agenerally accepted scaling theory has been developed to guide chipmanufacturers in the appropriate resize of all aspects of thesemiconductor device design at the same time, i.e., at each technologyor scaling node. The greatest impact of scaling on ion implantationprocess is the scaling of junction depths, which requires increasinglyshallow junctions as the device dimensions are decreased. Thisrequirement for increasingly shallow junctions as integrated circuittechnology scales translates into the following requirement: ionimplantation energies must be reduced with each scaling step. Theextremely shallow junctions called for by modern, sub-0.13 microndevices are termed “Ultra-Shallow Junctions”, or USJ.

Physical Limitations on Low-Energy Beam Transport

Due to the aggressive scaling of junction depths in CMOS processing, theion energy required for many critical implants has decreased to thepoint that conventional ion implantation systems, originally developedto generate much higher energy beams, deliver much reduced ion currentsto the wafer, reducing wafer throughput. The limitations of conventionalion implantation systems at low beam energy are most evident in theextraction of ions from the ion source, and their subsequent transportthrough the implanter's beam line. Ion extraction is governed by theChild-Langmuir relation, which states that the extracted beam currentdensity is proportional to the extraction voltage (i.e., beam energy atextraction) raised to the 3/2 power. FIG. 2 is a graph of maximumextracted boron beam current versus extraction voltage. For simplicity,an assumption has been made that only ¹¹B⁺ ions are present in theextracted beam. FIG. 2 shows that as the energy is reduced, extractioncurrent drops quickly. In a conventional ion implanter, this regime of“extraction-limited” operation is seen at energies less than about 10keV. Similar constraints affect the transport of the low-energy beamafter extraction. A lower energy ion beam travels with a smallervelocity, hence for a given value of beam current the ions are closertogether, i.e., the ion density increases. This can be seen from therelation J=ηev, where J is the ion beam current density in mA/cm², n isthe ion density in ions/cm⁻³, e is the electronic charge (=6.02×10⁻¹⁹Coulombs), and v is the average ion velocity in cm/s. In addition, sincethe electrostatic forces between ions are inversely proportional to thesquare of the distance between them, electrostatic repulsion is muchstronger at low energy, resulting in increased dispersion of the ionbeam. This phenomenon is called “beam blow-up”, and is the principalcause of beam loss in low-energy transport. While low-energy electronspresent in the implanter beam line tend to be trapped by thepositively-charged ion beam, compensating for space-charge blow-upduring transport, blow-up nevertheless still occurs, and is mostpronounced in the presence of electrostatic focusing lenses, which tendto strip the loosely-bound, highly mobile compensating electrons fromthe beam. In particular, severe extraction and transport difficultiesexist for light ions, such as the P-type dopant boron, whose mass isonly 11 amu. Being light, boron atoms penetrate further into thesubstrate than other atoms, hence the required implantation energies forboron are lower than for the other implant species. In fact, extremelylow implantation energies of less than 1 keV are being required forcertain leading edge USJ processes. In reality, most of the ionsextracted and transported from a typical BF₃ source plasma are not thedesired ion ¹¹B⁺, but rather ion fragments such as ¹⁹F⁺ and ⁴⁹BF₂ ⁺;these serve to increase the charge density and average mass of theextracted ion beam, further increasing space-charge blow-up. For a givenbeam energy, increased mass results in a greater beam purveyance; sinceheavier ions move more slowly, the ion density η increases for a givenbeam current, increasing space charge effects in accordance with thediscussion above.

Molecular Ion Implantation

One way to overcome the limitations imposed by the Child-Langmuirrelation discussed above is to increase the transport energy of thedopant ion by ionizing a molecule containing the dopant of interest,rather than a single dopant atom. In this way, while the kinetic energyof the molecule is higher during transport, upon entering the substrate,the molecule breaks up into its constituent atoms, sharing the energy ofthe molecule among the individual atoms according to their distributionin mass, so that the dopant atom's implantation energy is much lowerthan the original transport kinetic energy of the molecular ion.Consider the dopant atom “X” bound to a radical “Y” (disregarding forpurposes of discussion the issue of whether “Y” affects thedevice-forming process). If the ion XY⁺ were implanted in lieu of X⁺,then XY⁺ must be extracted and transported at a higher energy, increasedby a factor equal to the mass of XY divided by the mass of X; thisensures that the velocity of X in either case is the same. Since thespace-charge effects described by the Child-Langmuir relation discussedabove are super-linear with respect to ion energy, the maximumtransportable ion current is increased. Historically, the use ofpolyatomic molecules to ameliorate the problems of low energyimplantation is well known in the art. A common example has been the useof the BF₂ ⁺ molecular ion for the implantation of low-energy boron, inlieu of B⁺. This process dissociates BF₃ feed gas to the BF₂ ⁺ ion forimplantation. In this way, the ion mass is increased to 49 AMU, allowingan increase of the extraction and transport energy by more than a factorof 4 (i.e., 49/11) over using single boron atoms. Upon implantation,however, the boron energy is reduced by the same factor of (49/11). Itis worthy of note that this approach does not reduce the current densityin the beam, since there is only one boron atom per unit charge in thebeam. In addition, this process also implants fluorine atoms into thesemiconductor substrate along with the boron, an undesirable feature ofthis technique since fluorine has been known to exhibit adverse effectson the semiconductor device.

Cluster Implantation

In principle, a more effective way to increase dose rate than by the XY⁺model discussed above is to implant clusters of dopant atoms, that is,molecular ions of the form X_(n)Y_(m) ⁺, where n and m are integers andn is greater than one. Recently, there has been seminal work usingdecaborane as a feed material for ion implantation. The implantedparticle was a positive ion of the decaborane molecule, B₁₀H₁₄, whichcontains 10 boron atoms, and is therefore a “cluster” of boron atoms.This technique not only increases the mass of the ion and hence thetransport ion energy, but for a given ion current, it substantiallyincreases the implanted dose rate, since the decaborane ion B₁₀H_(x) ⁺has ten boron atoms. Importantly, by significantly reducing theelectrical current carried in the ion beam (by a factor of 10 in thecase of decaborane ions) not only are beam space-charge effects reduced,increasing beam transmission, but wafer charging effects are reduced aswell. Since positive ion bombardment is known to reduce device yields bycharging the wafer, particularly damaging sensitive gate isolation, sucha reduction in electrical current through the use of cluster ion beamsis very attractive for USJ device manufacturing, which must increasinglyaccommodate thinner gate oxides and exceedingly low gate thresholdvoltages. Thus, there is a critical need to solve two distinct problemsfacing the semiconductor manufacturing industry today: wafer charging,and low productivity in lowenergy ion implantation. As we will showlater in this document, the present invention proposes to furtherincrease the benefits of cluster implantation by using significantlylarger boron hydride clusters having n>10. In particular, we haveimplanted the B₁₈H_(x) ⁺ ion, and further propose to implant theB₃₆H_(x) ⁺ ion, using the solid feed material octadecaborane, or B₁₈H₂₂.We will present first results showing that this technology is asignificant advance over previous efforts in boron cluster implantation.

Ion Implantation Systems

Ion implanters have historically been segmented into three basiccategories: high current, medium current, and high energy implanters.Cluster beams are useful for high current and medium currentimplantation processes. In particular, today's high current implantersare primarily used to form the low energy, high dose regions of thetransistor such as drain structures and doping of the polysilicon gates.They are typically batch implanters, i.e., processing many wafersmounted on a spinning disk, the ion beam remaining stationary. Highcurrent transport systems tend to be simpler than medium currenttransport systems, and incorporate a large acceptance of the ion beam.At low energies and high currents, prior art implanters produce a beamat the substrate which tends to be large, with a large angulardivergence (e.g., a half-angle of up to seven degrees). In contrast,medium current implanters typically incorporate a serial (one wafer at atime) process chamber, which offers a high tilt capability (e.g., up to60 degrees from the substrate normal). The ion beam is typicallyelectromagnetically or electrodynamically scanned across the wafer at ahigh frequency, up to about 2 kiloHertz in one dimension (e.g.,laterally) and mechanically scanned at a low frequency of less than 1Hertz in an orthogonal direction (e.g., vertically), to obtain arealcoverage and provide dose uniformity over the substrate. Processrequirements for medium current implants are more complex than those forhigh current implants. In order to meet typical commercial implant doseuniformity and repeatability requirements of a variance of only a fewper cent, the ion beam must possess excellent angular and spatialuniformity (angular uniformity of beam on wafer of ≦1 deg, for example).Because of these requirements, medium current beam lines are engineeredto give superior beam control at the expense of reduced acceptance. Thatis, the transmission efficiency of the ions through the implanter islimited by the emittance of the ion beam. Presently, the generation ofhigher current (about 1 mA) ion beams at low (<10 keV) energy isproblematic in serial implanters, such that wafer throughput isunacceptably low for certain lower energy implants (for example, in thecreation of source and drain structures in leading edge CMOS processes).Similar transport problems also exist for batch implanters (processingmany wafers mounted on a spinning disk) at the low beam energies of <5keV per ion.

While it is possible to design beam transport optics which are nearlyaberration-free, the ion beam characteristics (spatial extent, spatialuniformity, angular divergence and angular uniformity) are nonethelesslargely determined by the emittance properties of the ion source itself(i.e., the beam properties at ion extraction which determine the extentto which the implanter optics can focus and control the beam as emittedfrom the ion source). The use of cluster beams instead of monomer beamscan significantly enhance the emittance of an ion beam by raising thebeam transport energy and reducing the electrical current carried by thebeam. However, prior art ion sources for ion implantation are noteffective at producing or preserving ionized clusters of the required N-and P-type dopants. Thus, there is a need for cluster ion and clusterion source technology in order to provide a better-focused, morecollimated and more tightly controlled ion beam on target, and inaddition to provide higher effective dose rates and higher throughputsin semiconductor manufacturing.

An alternative approach to beam line ion implantation for the doping ofsemiconductors is so-called “plasma immersion”. This technique is knownby several other names in the semiconductor industry, such as PLAD(PLAsma Doping), PPLAD (Pulsed PLAsma Doping, and PI³ (Plasma ImmersionIon Implantation). Doping using these techniques requires striking aplasma in a large vacuum vessel that has been evacuated and thenbackfilled with a gas containing the dopant of choice such as borontriflouride, diborane, arsine, or phosphine. The plasma by definitionhas positive ions, negative ions and electrons in it. The target is thenbiased negatively thus causing the positive ions in the plasma to beaccelerated toward the target. The energy of the ions is described bythe equation U=QV, where U is the kinetic energy of the ions, Q is thecharge on the ion, and V is the bias on the wafer. With this techniquethere is no mass analysis. All positive ions in the plasma areaccelerated and implanted into the wafer. Therefore extremely cleanplasma must be generated. With this technique of doping a plasma ofdiborane, phosphine or arsine gas is formed, followed by the applicationof a negative bias on the wafer. The bias can be constant in time,time-varying, or pulsed. Dose can be parametrically controlled byknowing the relationship between pressure of the vapor in the vessel,the temperature, the magnitude of the biasing and the duty cycle of thebias voltage and the ion arrival rate on the target. It is also possibleto directly measure the current on the target. While Plasma Doping isconsidered a new technology in development, it is attractive since ithas the potential to reduce the per wafer cost of performing low energy,high dose implants, particularly for large format (e.g., 300 mm) wafers.In general, the wafer throughput of such a system is limited by waferhandling time, which includes evacuating the process chamber and purgingand re-introducing the process gas each time a wafer or wafer batch isloader into the process chamber. This requirement has reduced thethroughput of Plasma Doping systems to about 100 wafers per hour (WPH),well below the maximum mechanical handling capability of beamline ionimplantation systems, which can process over 200 WPH.

Negative Ion Implantation

It has recently been recognized (see, for example, Junzo Ishikawa etal., “Negative-Ion Implantation Technique”, Nuclear Instruments andMethods in Physics Research B 96 (1995) 7-12) that implanting negativeions offers advantages over implanting positive ions. One very importantadvantage of negative ion implantation is to reduce the ionimplantation-induced surface charging of VLSI devices in CMOSmanufacturing. In general, the implantation of high currents (on theorder of 1 mA or greater) of positive ions creates a positive potentialon the gate oxides and other components of the semiconductor devicewhich can easily exceed gate oxide damage thresholds. When a positiveion impacts the surface of a semiconductor device, it not only depositsa net positive charge, but liberates secondary electrons at the sametime, multiplying the charging effect. Thus, equipment vendors of ionimplantation systems have developed sophisticated charge controldevices, so-called electron flood guns, to introduce low-energyelectrons into the positively-charged ion beam and onto the surface ofthe device wafers during the implantation process. Such electron floodsystems introduce additional variables into the manufacturing process,and cannot completely eliminate yield losses due to surface charging. Assemiconductor devices become smaller and smaller, transistor operatingvoltages and gate oxide thicknesses become smaller as well, reducing thedamage thresholds in semiconductor device manufacturing, furtherreducing yield. Hence, negative ion implantation potentially offers asubstantial improvement in yield over conventional positive ionimplantation for many leading-edge processes. Unfortunately, thistechnology is not yet commercially available, and indeed negative ionimplantation has not to the author's knowledge been used to fabricateintegrated circuits, even in research and development.

SUMMARY OF THE INVENTION

An object of this invention is to provide a method of manufacturing asemiconductor device, this method being capable of forming ultra-shallowimpurity-doped regions of P-type (i.e., acceptor) conductivity in asemiconductor substrate, and furthermore to do so with highproductivity.

Another object of this invention is to provide a method of manufacturinga semiconductor device, this method being capable of formingultra-shallow impurity-doped regions of P-type (i.e., acceptor)conductivity in a semiconductor substrate using ionized clusters of theform B_(n)H_(x) ⁺ and B_(n)H_(x) ⁻ where 10<n<100 and 0≦×≦n+4.

A further object of this invention is to provide a method ofmanufacturing a semiconductor device by implanting ionized molecules ofoctadecaborane, B₁₈H₂₂, of the form B₁₈H_(x) ⁺ or B₁₈H_(x) ⁺, where x isan integer less than or equal to 22.

A still further object of this invention is to provide for an ionimplantation system for manufacturing semiconductor devices, which hasbeen designed to form ultra shallow impurity-doped regions of either Nor P conductivity type in a semiconductor substrate through the use ofcluster ions.

According to one aspect of this invention, there is provided a method ofimplanting cluster ions comprising the steps of: providing a supply ofmolecules which each contain a plurality of dopant atoms into anionization chamber, ionizing said molecules into dopant cluster ions,extracting and accelerating the dopant cluster ions with an electricfield, selecting the desired cluster ions by mass analysis, modifyingthe final implant energy of the cluster ion through post-analysis ionoptics, and implanting the dopant cluster ions into a semiconductorsubstrate.

An object of this invention is to provide a method that allows thesemiconductor device manufacturer to ameliorate the difficulties inextracting low energy ion beams by implanting a cluster of n dopantatoms (n=18 in the case of B₁₈H_(x) ⁺) rather than implanting a singleatom at a time. The cluster ion implant approach provides the equivalentof a much lower energy monatomic implant since each atom of the clusteris implanted with an energy of approximately E/n. Thus, the implanter isoperated at an extraction voltage approximately n times higher than therequired implant energy, which enables higher ion beam current,particularly at the low implantation energies required by USJ formation.In addition, each milliamp of cluster current provides the equivalent of18 mA of monomer boron. Considering the ion extraction stage, therelative improvement in transport efficiency enabled by cluster ionimplant can be quantified by evaluating the Child-Langmuir limit. It isrecognized that this limit can be approximated by:J _(max)=1.72(Q/A)^(1/2) V ^(3/2) d ⁻²,  (1)where J_(max) is in mA/cm², Q is the ion charge state, A is the ion massin AMU, V is the extraction voltage in kV, and d is the gap width in cm.FIG. 2 is a graph of equation (1) for the case of ¹¹B+with d=1.27 cm,where the average mass of the extracted beam is assumed to be 15 AMU. Inpractice, the extraction optics used by many ion implanters can be madeto approach this limit. By extension of equation (1), the followingfigure of merit, Δ, can be defined to quantify the increase inthroughput, or implanted dose rate, for a cluster ion implant relativeto monatomic implantation:Δ=n(U _(n) /U ₁)^(3/2)(M _(n) /M ₁)^(−1/2).  (2)

Here, Δ is the relative improvement in dose rate (atoms/sec) achieved byimplanting a cluster with n atoms of the dopant of interest at an energyUn relative to the single atom implant of an atom of mass m₁ at energyU₁, where U_(i)=eV. In the case where U_(n) is adjusted to give the samedopant implantation depth as the monatomic (n=1) case, equation (2)reduces to:Δ=n².  (3)

Thus, the implantation of a cluster of n dopant atoms has the potentialto provide a dose rate n² higher than the conventional implant of singleatoms. In the case of B₁₈H_(x), this maximum dose rate improvement ismore than 300. The use of cluster ions for ion implant clearly addressesthe transport of low energy (particularly sub-keV) ion beams. It is tobe noted that the cluster ion implant process only requires oneelectrical charge per cluster, rather than having every dopant atomcarrying one electrical charge, as in the conventional case. Thetransport efficiency (beam transmission) is thus improved, since thedispersive Coulomb forces are reduced with a reduction in chargedensity. Importantly, this feature enables reduced wafer charging, sincefor a given dose rate, the electrical beam current incident on the waferis dramatically reduced. Also, since the present invention producescopious amounts of negative ions of boron hydrides, such as B₁₈H_(x) ⁻,it enables the commercialization of negative ion implantation at highdose rates. Since negative ion implantation produces less wafer chargingthan positive ion implantation, and since these electrical currents arealso much reduced through the use of clusters, yield loss due to wafercharging can be further reduced. Thus, implanting with clusters of ndopant atoms rather than with single atoms ameliorates basic transportproblems in low energy ion implantation and enables a dramatically moreproductive process.

Enablement of this method requires the formation of the cluster ions.The prior art ion sources used in commercial ion implanters produce onlya small fraction of primarily lower-order (e.g., n=2) clusters relativeto their production of monomers, and hence these implanters cannoteffectively realize the low energy cluster beam implantation advantageslisted above. Indeed, the intense plasmas provided by many conventionalion sources rather dissociate molecules and clusters into theircomponent elements. The novel ion source described herein producescluster ions in abundance due to its use of a “soft” ionization process,namely electron-impact ionization. The ion source of the presentinvention is designed expressly for the purpose of producing andpreserving dopant cluster ions. Instead of striking an arc dischargeplasma to create ions, the ion source of the present invention useselectron-impact ionization of the process gas by electrons injected inthe form of one or more focused electron beams.

DESCRIPTION OF THE DRAWINGS

These and other advantages of the present invention will be readilyunderstood with reference to the following specification and attacheddrawing wherein:

FIG. 1A is a schematic diagram of an exemplary high-current cluster ionimplantation system in accordance with the present invention.

FIG. 1B is a schematic diagram of the accel-decel electrode used in theimplantation system of FIG. 1A.

FIG. 1C is an alternative embodiment of a high-current cluster ionimplantation system in accordance with the present invention.

FIG. 1D is yet another alternative embodiment of a high-current clusterion implantation system in accordance with the present invention.

FIG. 1E is a schematic diagram of an exemplary medium-current clusterion implantation system in accordance with the present invention.

FIG. 2 is a graphical diagram illustrating maximum ¹¹B⁺ beam current vs.extraction energy according to the Child-Langmuir Law of equation (1).

FIG. 3 is a perspective view of an ion source in accordance with thepresent invention, shown in cutaway to expose internal components.

FIG. 4A is a side view of a portion of one embodiment of the ion sourceshown in FIG. 3, shown in cutaway with the electron beam and magneticfields shown superimposed thereupon.

FIG. 4B is similar to FIG. 4A but illustrates an alternativeconfiguration with two electron beam sources.

FIG. 5A is a perspective diagram of the cluster ion source of FIG. 3,showing details of the ionization region.

FIG. 5B is similar to FIG. 5A but illustrates an alternativeconfiguration with two electron beam sources.

FIG. 5C is a simplified top view of the electron beam forming region ofthe ion source illustrated in FIG. 5B.

FIG. 6 is a diagram of the 3-zone temperature control system used in theion source of the present invention.

FIG. 7A is a perspective view of the magnetic yoke assembly,illustrating the magnetic circuit which includes permanent magnets.

FIG. 7B is a perspective view of the magnetic yoke assembly integratedinto the ionization chamber of the ion source of the present invention.

FIG. 7C is an illustration of the magnetic flux through a cross-sectionof the magnetic yoke assembly in the xy plane.

FIG. 7D is a perspective view of an alternative embodiment of themagnetic yoke assembly illustrated in FIG. 7A, which includes anelectromagnet.

FIG. 7E is similar to FIG. 7B except that it relates to the embodimentillustrated in FIG. 7D.

FIG. 7F is an illustration of the magnetic flux through a cross-sectionof the magnetic yoke assembly depicted in FIG. 7E, in the yz plane.

FIG. 7G is similar to FIG. 7F except that it illustrates magnetic fluxin the nz plane.

FIG. 7H depicts the ion source of the present invention with ahigh-permeability magnetic shield between the yoke assembly of FIG. 7Band the electron gun.

FIG. 8A is a graphical illustration of octadecaborane beam current andvapor pressure, versus vaporizer temperature, using the ion source ofthe present invention.

FIG. 8B is a ball-and-stick model of the B₁₈H₂₂ molecule.

FIG. 9 is a graphical illustration of the positive ion mass spectrum ofB₁₈H₂₂ generated with the ion source of the present invention, collectedat high mass resolution.

FIG. 10 is a graphical illustration of the negative ion mass spectrum ofB₁₈H₂₂ overlaid with a positive ion mass spectrum of B₁₈H₂₂, bothcollected at high mass resolution, and generated with the ion source ofthe present invention.

FIG. 11A is a graphical illustration of the positive ion mass spectrumof B₁₈H₂₂ generated with the ion source of the present invention,collected at low mass resolution.

FIG. 11B is a graphical illustration of the positive mass spectrum ofB₁₈H₂₂ generated with the ion source of the present invention, collectedat highest mass resolution and with an expanded horizontal scale, sothat individual ion masses can be resolved.

FIG. 12 is a graphical illustration of B₁₈H_(x) ⁺ beam current as afunction of beam extraction energy, measured near the wafer position bya cluster ion implantation system of the present invention.

FIG. 13 is a graphical illustration of the data of FIG. 12 converted toboron dose rate (using B₁₈H_(x) ⁺ implantation) as a function of boronimplant energy, using a cluster ion implantation system of the presentinvention.

FIG. 14 is a diagram of a CMOS fabrication sequence during formation ofthe NMOS drain extension.

FIG. 15 is a diagram of a CMOS fabrication sequence during formation ofthe PMOS drain extension.

FIG. 16 is a diagram of a semiconductor substrate in the process ofmanufacturing a NMOS semiconductor device, at the step of N-type drainextension implant.

FIG. 17 is a diagram of a semiconductor substrate in the process ofmanufacturing a NMOS semiconductor device, at the step of thesource/drain implant.

FIG. 18 is a diagram of a semiconductor substrate in the process ofmanufacturing an PMOS semiconductor device, at the step of P-type drainextension implant.

FIG. 19 is a diagram of a semiconductor substrate in the process ofmanufacturing a PMOS semiconductor device, at the step of thesource/drain implant.

FIG. 20 is a graphical illustration of as-implanted SIMS profiles ofboron concentrations from a 20 keV B₁₈H_(x) ⁺ ion beam implanted into asilicon wafer by a cluster ion implantation system of the presentinvention.

FIG. 21 is a graphical illustration of the ionization cross-section a asa function of electron energy T for ammonia (NH₃).

DETAILED DESCRIPTION

Cluster Ion Implantation System

FIG. 1A is a schematic diagram of a cluster ion implantation system ofthe high current type in accordance with the present invention.Configurations other than that shown in FIG. 1A are possible. Ingeneral, the electrostatic optics of ion implanters employ slots(apertures displaying a large aspect ratio in one dimension) embedded inelectrically conductive plates held at different potentials, which tendto produce ribbon beams, i.e., beams which are extended in onedimension. This approach has proven effective in reducing space-chargeforces, and simplifies the ion optics by allowing the separation offocusing elements in the dispersive (short axis) and non-dispersive(long axis) directions. The cluster ion source 10 of the presentinvention is coupled with an extraction electrode 220 to create an ionbeam 200 which contains cluster ions, such as B₁₈H_(x) ⁺ or As₄ ⁺. Theions are extracted from an elongated slot in ion source 10, called theion extraction aperture, by an extraction electrode 220, which alsoincorporates slot lenses of somewhat larger dimension than those of theion extraction aperture; typical dimensions of the ion extractionaperture may be, for example, 50 mm tall by 8 mm wide, but otherdimensions are possible. The electrode is an accel-decel electrode in atetrode configuration, i.e., the electrode extracts ions from the ionsource at a higher energy and then decelerates them prior to theirexiting the electrode.

A schematic diagram of the accel-decel electrode is shown in FIG. 1 B.It is comprised of suppression plate 300 biased by power supply Vs,extraction plate 302 biased by power supply Vf, and ground plate 304,which is at implanter terminal ground (not necessarily earth ground in adecel machine). Ion extraction aperture plate 80 is held unipotentialwith ionization chamber 44 of ion source 10, which is held at ion sourcepotential by power supply Va. For the production of positive ions, Va>0,Vf<0, and Vs<0. For production of negative ions, Va<0, Vf=0, and Vs>0.For example, to produce 20 keV positive ions, typical voltages would beVa=20 kV, Vs=−5 kV, Vf=−15 kV. Note that this means that the actualvoltages of the various plates are: extraction aperture plate 80=20 kV,suppression plate 300=−20 kV, extraction plate 302=−15 kV, ground plate304=0V. For producing negative ions, the power supply voltages arereversed. By using bipolar power supplies, either negative or positiveions may be produced by the novel implanter designs of FIG. 1A, 1C, 1Dand 1E. Thus, ions are extracted at higher energy from the ion source,and are decelerated upon leaving the ground plate 304, enabling higherextracted currents and improved focusing and transmission of theresultant ion beam 200.

The ion beam 200 (FIG. 1A) typically contains ions of many differentmasses, i.e., all of the ion species of a given charge polarity createdin the ion source 210. The ion beam 200 then enters an analyzer magnet230. The analyzer magnet 230 creates a dipole magnetic field within theion beam transport path as a function of the current in the magnetcoils; the direction of the magnetic field is shown as normal to theplane of FIG. 1A, which is also along the non-dispersive axis of theone-dimensional optics. The analyzer magnet 230 is also a focusingelement which forms a real image of the ion extraction aperture (i.e.,the optical “object” or source of ions) at the location of the massresolving aperture 270. Thus, mass resolving aperture 270 has the formof a slot of similar aspect ratio but somewhat larger dimension than theion extraction aperture. In one embodiment, the width of resolvingaperture 270 is continuously variable to allow selection of the massresolution of the implanter. This feature is important for maximizingdelivered beam current of boron hydride cluster ions, which display anumber of ion states separated by one AMU, as for example is illustratedin FIG. 11A. A primary function of the analyzer magnet 230 is tospatially separate, or disperse, the ion beam into a set of constituentbeamlets by bending the ion beam in an arc whose radius depends on themass-to-charge ratio of the discrete ions. Such an arc is shown in FIG.1A as a beam component 240, the selected ion beam. The analyzer magnet230 bends a given beam along a radius given by Equation (4) below:R=(2mU)^(1/2) /qB,  (4)where R is the bending radius, B is the magnetic fux density, m is theion mass, U is the ion kinetic energy and q is the ion charge state.

The selected ion beam is comprised of ions of a narrow range ofmass-energy product only, such that the bending radius of the ion beamby the magnet sends that beam through mass resolving aperture 270. Thecomponents of the beam that are not selected do not pass through themass-resolving aperture 270, but are intercepted elsewhere. For beamswith smaller mass-to-charge ratios m/q 250 than the selected beam 240,for example comprised of hydrogen ions having a mass of 1 or 2 AMU, themagnetic field induces a smaller bending radius and the beam interceptsthe inner radius wall 300 of the magnet vacuum chamber, or elsewhereupstream of the mass resolving aperture. For beams with largermass-to-charge ratios 260 than the selected beam 240, the magnetic fieldinduces a larger bending radius, and the beam strikes the outer radiuswall 290 of the magnet chamber, or elsewhere upstream of the massresolving aperture. As is well established in the art, the combinationof analyzer magnet 230 and mass resolving aperture 270 form a massanalysis system which selects the ion beam 240 from the multi-speciesbeam 200 extracted from the ion source 10. The selected beam 240 thenpasses through a post-analysis acceleration/deceleration electrode 310.This stage 310 can adjust the beam energy to the desired final energyvalue required for the specific implantation process. For example, inlow-energy, high-dose process higher currents can be obtained if the ionbeam is formed and transported at a higher energy and then deceleratedto the desired, lower implant ion energy prior to reaching the wafer.The post-analysis acceleration/deceleration lens 310 is an electrostaticlens similar in construction to decel electrode 220. To producelow-energy positive ion beams, the front portion of the implanter isenclosed by terminal enclosure 208 and floated below earth ground. Agrounded Faraday cage 205 surrounds the enclosure 208 for safetyreasons. Thus, the ion beam can be transported and mass-analyzed athigher energies, and decelerated prior to reaching the workpiece. Sincedecel electrode 300 is a strong-focusing optic, dual quadrupoles 320refocus ion beam 240 to reduce angular divergence and spatial extent. Inorder to prevent ions which have undergone charge-exchange orneutralization reactions between the resolving aperture and thesubstrate 312 (and therefore do not possess the correct energy) frompropagating to substrate 312, a neutral beam filter 310 a (or “energyfilter”) is incorporated within this beam path. For example, the neutralbeam filter 310 a shown incorporates a “dogleg” or small-angledeflection in the beam path which the selected ion beam 240 isconstrained to follow through an applied DC electromagnetic field; beamcomponents which have become electrically neutral or multiply-charged,however, would necessarily not follow this path. Thus, only the ion ofinterest and with the correct ion energy is passed downstream of theexit aperture 314 of the filter 310 a.

Once the beam is shaped by a quadrupole pair 320 and filtered by aneutral beam filter 310 a, the ion beam 240 enters the wafer processchamber 330, also held in a high vacuum environment, where it strikesthe substrate 312 which is mounted on a spinning disk 315. Variousmaterials for the substrate are suitable with the present invention,such as silicon, silicon-on-insulator strained superlattice substrateand a silicon germanium (SiGe) strained superlattice substrate. Manysubstrates may be mounted on the disk so that many substrates may beimplanted simultaneously, i.e., in batch mode. In a batch system,spinning of the disk provides mechanical scanning in the radialdirection, and either vertical or horizontal scanning of the spinningdisk is also effected at the same time, the ion beam remainingstationary.

Altemative embodiments of high-current implanters are illustrated inFIG. 1C and FIG. 1D. In particular, FIG. 1C illustrates an accel-decelimplanter similar to that described in FIG. 1A, except that the beamline has been significantly shortened by removal of dual quadrupoles 320and neutral beam filter 310 a. This configuration results in better beamtransmission through the implanter, and provides for higher beamcurrents on substrate 312.

FIG. 1D illustrates a non-accel-decel implanter, i.e., in which thevacuum system of the entire implanter is at earth ground. Thus, in FIG.1D the decel lens 310 and terminal enclosure 208 are deleted relative tothe embodiment shown in FIG. 1C. The method of cluster beam implantationdelivers very high effective dopant beam currents at sub-keV energies,even without deceleration. The cluster beam implantation systemillustrated in FIG. 1D is greatly simplified and more economical toproduce. It also has a shorter beam line, thus increasing thetransmission of the beam to the substrate 312.

FIG. 1E schematically illustrates a proposed medium current implanterwhich incorporates the present invention. There are many alternativeconfigurations from that which is shown in FIG. 1E. Ion beams typicallya few centimeters high and less than one centimeter wide are produced inthe ion source 400 extracted by the extraction electrode 401 andtransported through the analyzer magnet 402 and mass resolving aperture403. This produces a beam 404 of a specific massenergy product. Sincethe energy is fixed by the extraction voltage, typically a single masspasses through the mass analyzer and resolving aperture at a givenanalyzer magnet 402 field. Equation (4) above describes this process.The boron hydride cluster ion beam exits the mass resolving aperture andenters the accel-decel electrode 405. This electrode is specificallydesigned to either add energy to the ion beam or reduce the energy ofthe ion beam. For low energy implants beam transport is enhanced byextracting the beam at a higher energy and then reducing the energy inthe deceleration electrode. The Child-Langmuir Law, as illustrated inFIG. 2, limits the current that can be extracted from the ion source.The U^(3/2) dependence of current density limit on energy, where U isthe extraction energy, is responsible for increased current at higherextraction energies. For higher energy implants the accel-decelelectrode is used to increase the energy of the ion beam to an energythat is above the extraction energy. Extraction energies are typically20-40 keV, and can decelerated to less than one keV or accelerated toenergies as high as 200 keV for singly charged ions, and as high as 500keV for multiply charged ions. After acceleration, the beam istransported into a quadrupole lens 406 to refocus the beam after theenergy is adjusted by the accel-decel electrode. This step increases thetransmission efficiency through the rest of the implanter. If the beamis allow to expand upon leaving the accel-decel region it will hit thewalls of beam line and cause particles to be generated by the beamstriking the wall of the beam line 408 as well as not being availablefor implantation into the target. Next the beam encounters the scanningmodule 407, which scans the beam in one dimension, typicallyhorizontally. The scan frequency is often in the kilo Hertz range. Thiscauses the beam to have a very large angular variation, resulting in thebeam striking the target at different angles on different parts of thetarget. To eliminate this scan induced divergence the beam is directedthrough a beam collimator 410. Beam collimators are either magnetic orelectrostatic and yield a wide parallel beam 409. The collimator alsoremoves ions from the beam which are at a different energy thanintended, due to charge-exchange reactions encountered in the beam line.Upon exiting the collimator the beam enters the wafer process chamber411 and strikes the target 412. Medium current implanters usuallyprocess one wafer at a time. This is known in the industry as serialprocessing. Areal coverage of the wafer is accomplished by translatingthe wafer in a direction orthogonal to the direction of the beam sweep,for example, in the vertical dimension. The frequency of the vertical isvery slow compared to the “fast” scan frequency, having a period of 5-10or more seconds per cycle. The dose (ions/cm²) on the wafer iscontrolled by monitoring the beam current in a Faraday cup 413 mountednext to the wafer. Once each scan, at the extreme end of the scan, thebeam enters the Faraday cup and is monitored. This allows the beamcurrent to be measured at a rate equal to the scan frequency of thebeam, for example 1000 times each second. This signal is then used tocontrol the vertical translation speed in the orthogonal direction tobeam scan to obtain a uniform dose across the wafer. In addition, theserial process chamber allows for freedom to orient the wafer relativeto the ion beam. Wafers can be rotated during the implant process, andcan be by tilted to large angles, as much as 60 degrees to the beamnorrnal.

The use of cluster ion beams such as B₁₈H_(x) ⁺ or As₄H_(x) ⁺ allow thebeam extraction and transmission to take place at higher energies thanwould be the case for monomers such as B⁺ or As⁺. Upon striking thetarget, the ion energy is partitioned by mass ratio of the individual,constituent atoms. For B₁₈H₂₂ the effective boron energy is 10.8/216.4of the beam energy, because an average boron atom has a mass of 10.8 amuand the molecule has an average mass of 216.4 amu. This allows the beamto be extracted and transported at 20 times the implant energy.Additionally the dose rate is 18 times higher than for a monomer ion.This results in higher throughput and less charging of the wafer. Wafercharging is reduced because there is only one charge for 18 atomsimplanted into the wafer instead of one charge for every atom implantedwith a monomer beam.

Plasma Doping with Clusters

An alternative approach to beam line ion implantation for the doping ofsemiconductors is so-called “plasma immersion”. This technique is knownby several other names in the semiconductor industry, such as PLAD(PLAsma Doping), PPLAD (Pulsed PLAsma Doping, and PI³ (Plasma ImmersionIon Implantation). Doping using these techniques requires striking aplasma in a large vacuum vessel that has been evacuated and thenbackfilled with a gas containing the dopant of choice such as borontriflouride, diborane, arsine, or phosphine. The plasma by definitionhas positive ions, negative ions and electrons in it. The target is thenbiased negatively thus causing the positive ions in the plasma to beaccelerated toward the target. The energy of the ions is described bythe equation U=QV, where U is the kinetic energy of the ions, Q is thecharge on the ion, and V is the bias on the wafer. With this techniquethere is no mass analysis. All positive ions in the plasma areaccelerated and implanted into the wafer. Therefore extremely cleanplasma must be generated. With this technique of doping a vapor of boronclusters such as B₁₈H₂₂, or arsenic clusters such as As₄H_(x) can beintroduced into the vessel and a plasma ignited, followed by theapplication of a negative bias on the wafer. The bias can be constant intime, time-varying, or pulsed. The use of these clusters will bebeneficial since the ratio of dopant atoms to hydrogen (e.g., usingB₁₈H₂₂ versus B₂H₆ and As₄H_(x) versus AsH₃) is greater for hydrideclusters than for simple hydrides, and also the dose rates can be muchhigher when using clusters. Dose can be parametrically controlled byknowing the relationship between pressure of the vapor in the vessel,the temperature, the magnitude of the biasing and the duty cycle of thebias voltage and the ion arrival rate on the target. It is also possibleto directly measure the current on the target. As with beam lineimplantation, using octadecaborane would yield an 18 times enhancementin dose rate and 20 times higher accelerating voltages required ifoctadecaborane were the vapor of choice. If As₄H_(x) were used therewould be a four times dose rate enhancement and a four times the voltagerequired. There would also be reduced changing as with the beam lineimplants utilizing clusters.

Cluster Ion Source

FIG. 3 is a diagram of a cluster ion source 10 and its variouscomponents. The details of its construction, as well as its preferredmodes of operation, are disclosed in detail in commonly-owned U.S.patent application Ser. No. 10/183,768, “Electron Impact Ion Source”,submitted Jun. 26, 2002, inventor T. N. Horsky, herein incorporated byreference. The ion source 10 is one embodiment of a novel electronimpact ionization source. FIG. 3 is a cross-sectional schematic diagramof the source construction which serves to clarify the functionality ofthe components which make up the ion source 10. The ion source 10 ismade to interface to an evacuated vacuum chamber of an ion implanter orother process tool by way of a mounting flange 36. Thus, the portion ofthe ion source 10 to the right of flange 36, shown in FIG. 3, is at highvacuum (pressure <1×10⁻⁴ Torr). Gaseous material is introduced intoionization chamber 44 in which the gas molecules are ionized by electronimpact from electron beam 70A or 70B, which enters the ionizationchamber 44 through electron entrance aperture 71B such that electronbeam 70A or 70B is aligned with ion extraction aperture 81, and exitsionization chamber 44 through electron exit aperture 71A. In oneembodiment incorporating a single electron gun and a beam dump, shown inFIG. 4A and FIG. 5A, after leaving ionization chamber 44, the electronbeam is stopped by beam dump 72 located external to ionization chamber44. Thus, ions are created adjacent to the ion extraction aperture 81,which appears as a slot in the ion extraction aperture plate 80. Theions are then extracted and formed into an energetic ion beam by anextraction electrode (not shown) located in front of the ion extractionaperture plate 80. The ionization region is shown in more detail inFIGS. 4A and 4B and in FIGS. 5A and 5B.

Referring now to FIG. 3, gases may be fed into the ionization chamber 44via a gas conduit 33. Solid feed materials can be vaporized in avaporizer 28, and the vapor fed into the ionization chamber 44 through avapor conduit 32 within the source block 35. Solid feed material 29,located under a perforated separation barrier 34 a, is held at a uniformtemperature by temperature control of the vaporizer housing 30. Vapor 50which accumulates in a ballast volume 31 feeds through conduit 39 andthrough one or more shutoff valves 100 and 110. The nominal pressure ofvapor 50 within shutoff valve 110 is monitored by capacitance manometergauge 60. The vapor 50 feeds into the ionization chamber 44 through avapor conduit 32, located in the source block 35. Thus, both gaseous andsolid dopant-bearing materials may be ionized by this ion source.

FIGS. 4A, 4B, 5A and 5B illustrate alternative embodiments of theoptical design of the ion source. In particular, FIGS. 4A and 5Aillustrate one embodiment of the invention incorporating a singleelectron source. FIGS. 4B and 5B illustrate an alternative embodimentincorporating dual electron sources.

Single Electron Source

In particular, FIG. 4A is a cross-sectional side view which illustratesone embodiment of the optical design of the ion source configuration inaccordance with the present invention. In this embodiment of theinvention, an electron beam 70 is emitted from a heated filament 110 andexecutes a 90 degree trajectory due to the influence of beam steerers,for example, incorporating a static magnetic field B 135 (in a directionnormal to the plane of the paper as indicated) into the ionizationchamber 44, passing first through base plate aperture 106 in base plate105, and then through electron entrance aperture 70 a in ionizationchamber 44. Electrons passing all the way through ionization chamber 44(i.e., through electron entrance aperture 70 a and electron exitaperture 71) are intercepted by a beam dump 72. Emitter shield 102 isunipotential with base plate 105 and provides electrostatic shieldingfor the propagating electron beam 70. As electron beam 70 propagatesthrough the base plate aperture 106, it is decelerated prior to enteringionization chamber 44 by the application of a voltage Va to base plate105 (provided by positive-going power supply 115), and voltage Vc to thefilament 135 (provided by negative-going power supply 116), both biasedrelative to the ionization chamber 44. It is important to maintain anelectron beam energy significantly higher than typically desired forionization in the beam-forming and the transport region, i.e., outsideof ionization chamber 44. This is due to the space charge effects whichseverely reduce the beam current and enlarge the electron beam diameterat low energies. Thus, it is desired to maintain the electron beamenergy between about 1.5 keV and 5 keV in this region.

Voltages are all relative to the ionization chamber 44. For example, ifVc=−0.5 kV and Va=1.5 kV, the energy of the electron beam is thereforegiven by e(Va−Vc), where e is the electronic charge (6.02×10⁻¹⁹Coulombs). Thus, in this example, the electron beam 70 is formed anddeflected at an energy of 2 keV, but upon entering electron entranceaperture 70 a, it has an energy of only 0.5 keV.

Other elements shown in FIG. 4A include an extracted ion beam 120, asource electrostatic shield 101, and emitter shield 102. Emitter shields102 shields the electron beams 70 from fields associated with thepotential difference between base plate 105 and the source shield 101,which is unipotential with ionization chamber 44. The source shield 101shields the ion beam 120 from fields generated by the potentialdifference between base plate 105 and ionization chamber 44, and alsoacts to absorb stray electrons and ions which may otherwise impact theion source elements. For this reason, emitter shields 102 and the sourceshield 101 are constructed of refractory metal, such as molybdenum.Alternatively, more complete shielding of the ion beam 120 from magneticfields B 135 and B′ 119 may be accomplished by fabricating source shield101 of a ferromagnetic substance, such as magnetic stainless steel.

FIG. 5A is a cutaway view illustrating the mechanical detail and whichshows explicitly how the contents of FIG. 4A are incorporated into theion source of FIG. 3. Electrons are thermionically emitted from filament110 and accelerated to anode 140, forming electron beam 70. Sinceelectron beam 70 is generated external to the ionization chamber, theemitter life is extended relative to known configurations, since theemitter is in the low-pressure environment of the implanter vacuumhousing in which the ion source resides, and since the emitter is alsoeffectively protected from ion bombardment.

Magnetic flux from permanent magnet 130 and magnetic pole assembly 125is used to steer the beam by establishing a uniform magnetic fieldacross the air gap between the ends of the magnetic pole assembly 125,wherein the electron beam 70 propagates. The magnetic field B 135 andthe electron beam energies of electron beam 70 are matched such thatelectron beam 70 is deflected through approximately 90 degrees, andpasses into ionization chamber 44 as shown. By deflecting electron beam70 for example, through 90 degrees, no line of sight exists betweenemitter 110 and ionization chamber 44 which contains the ions, thuspreventing bombardment of the emitters by energetic charged particles.

Since Va is positive relative to the ionization chamber 44, electronbeam 70 is decelerated as it passes through the gap defined by baseplate aperture 106 and electron entrance aperture 70 a. Thus, thecombination of base plate aperture 106 and electron entrance aperture 70a and the gap between them, forms an electrostatic lens, in this case, adecelerating lens. The use of a decelerating lens allows the ionizationenergy of the electron beam to be adjusted without substantiallyaffecting the electron beam generation and deflection.

The gap may be established by one or more ceramic spacers 132, whichsupport base plate 105 and act as a stand off from source block 35,which is at ionization chamber potential. The ceramic spacers 132provide both electrical isolation and mechanical support. Note that forclarity, the emitter shields 102 and the source shield 101 are not shownin FIG. 5A. Also not shown is the magnetic yoke assembly which is shownin FIG. 7A-7H.

Since the electron entrance aperture 106 can limit transmission ofelectron beam 70, base plate 105 can intercept a significant portion ofthe energetic electron beam 70. base plate 105 must therefore be eitheractively cooled, or passively cooled. Active cooling may be accomplishedby passing liquid coolant, such as water, through base plate 105, orforcing compressed air to flow through said base plate 105. In analternative embodiment, passive cooling is accomplished by allowing baseplate 105 to reach a temperature whereby they cool through radiation totheir surroundings. This steady-state temperature depends on theintercepted beam power, the surface area and emissivity of the baseplate, and the temperatures of surrounding components. Allowing the baseplate 105 to operate at elevated temperature, for example at 250C, isadvantageous when running condensable gases which can form contaminatingand particle-forming films on exposed cold surfaces.

Dual Electron Source

FIG. 4B is an alternative embodiment of the optical design illustratinga dual electron-beam ion source configuration. In this embodiment of theinvention, a pair of spatially separate electron beams 70 a and 70 b areemitted from a pair of spacially separate heated filaments 110 a and 110b and execute 90 degree trajectories due to the influence of beamsteerers or static magnetic fields B 135 a and 135 b (in a directionnormal to the plane of the paper as indicated) into the ionizationchamber 44, passing first through a pair of base plate apertures 106 aand 106 b and a pair of spaced apart base plates 105 a and 105 b, andthen through a pair of electron entrance apertures 71 a and 71 b.Electrons passing all the way through the ionization chamber 44 (i.e.,through both of the electron entrance apertures 71 a and 71 b) are benttoward a pair of emitter shields 102 a and 102 b by the beam steerers,or static magnetic fields 135 a and 135 b. As the electron beamspropagate through the base plate apertures 106 a and 106 b, they aredecelerated prior to entering ionization chamber 44 by the applicationof a voltage Va to the base plates 105 a and 105 b (provided bypositive-going power supply 115), and voltage Ve to the filaments 135 aand 135 b (provided by negative-going power supply 116). It is importantto maintain electron beam energies significantly higher than typicallydesired for ionization in the beam-forming and the transport region,i.e., outside of ionization chamber 44. This is due to the space chargeeffects which severely reduce the beam current and enlarge the electronbeam diameter at low energies. Thus, it is desired to maintain theelectron beam energies between about 1.5 keV and 5 keV in this region.

Similar to the embodiment for a single electron source, the voltages fora dual electron source are also all relative to the ionization chamber44. For example, if Ve=−0.5 kV and Va=1.5 kV, the energy of the electronbeam is therefore given by e(Va−Ve), where e is the electronic charge(6.02×10⁻¹⁹ Coulombs). Thus, in this example, the electron beam 70 a, 70b is formed and deflected at an energy of 2 keV, but upon enteringelectron entrance aperture 71a, 71 b it has an energy of only 0.5 keV.

The following table gives approximate values of magnetic field Brequired to bend an electron beam with energy E through 90 degrees.TABLE 1 Dependence of Magnetic Field Strength on Electron Energy toAccomplish a 90 Degree Deflection in the Present Invention ElectronEnergy E Magnetic Field B 1500 eV 51 G 2000 eV 59 G 2500 eV 66 G

Other elements shown in FIG. 4B include an extracted ion beam 120 a, asource electrostatic shield 101 a, and a pair of emitter shields 102 aand 102 b. These emitter shields 102 a and 102 b serve two purposes: toprovide shielding from electromagnetic fields, and to provide shieldingfrom stray electron or ion beams. For example, the emitter shields 102 aand 102 b shield the electron beams 70 a and 70 b from fields associatedwith the potential difference between base plates 105 a and 105 b andthe source shield 101, and also acts as a dump for stray electron beamsfrom the opposing electron emitter. The source shield 101 shields theion beam 120 from fields generated by the potential difference betweenbase plates 105 a and 105 b and the ionization chamber 44, and also actsto absorb stray electrons and ions which would otherwise impact the ionsource elements. For this reason, both of the emitter shields 102 a and102 b, as well as the source shield 101, are constructed of refractorymetal, such as molybdenum or graphite. Alternatively, more completeshielding of ion beam 120 a from the magnetic fields B 135 a and 135 bmay be accomplished by constructing the source shield 101 a of aferromagnetic substance, such as magnetic stainless steel.

FIG. 5B is a cutaway view illustrating the mechanical detail and whichshows explicitly how the contents of FIG. 4B are incorporated into theion source of FIG. 3. Electrons are thermionically emitted from one ormore of the filaments 110 a and 110 b and accelerated to a pair ofcorresponding anodes 140 a and 140 b forming the electron beams 70 a and70 b. Such a configuration offers several benefits. First, the filaments110 a and 110 b can be operated separately or together. Second, sincethe electron beams 70 a, 70 b are generated external to the ionizationchamber, the emitter life is extended relative to known configurations,since the emitter is in the low-pressure environment of the implantervacuum housing in which the ion source resides, and since the emitter isalso effectively protected from ion bombardment.

Magnetic flux from a pair of permanent magnets 130 a and 130 b and apair of magnetic pole assemblies 125 a and 125 b is used to form beamsteerers used to establish uniform magnetic fields across the air gapbetween the ends of the magnetic pole assemblies 125 a, 125 b, whereinthe electron beam 70 a, 70 b propagates. The magnetic fields 135 a and135 b and the electron beam energies of electron beams 70 a and 70 b arematched such that electron beams 70 a and 70 b are deflected 90 degrees,and pass into the ionization chamber 44 as shown. By deflecting theelectron beams 70 a and 70 b, for example, through 90 degrees, no lineof sight exists between the emitters and the ionization chamber 44 whichcontains the ions, thus preventing bombardment of the emitters byenergetic charged particles.

Since Va is positive relative to ionization chamber 44, the electronbeams 70A, 70B are decelerated as they pass through the gap defined bybase plate apertures 106 a and 106 b and the electron entrance apertures71 a and 71 b. Thus, the combination of base plate aperture 106 a andelectron entrance aperture 71 a, and baseplate aperture 106 b andelectron entrance aperture 71 b, and the gaps between them, each formsan electrostatic lens, in this case, a decelerating lens. The use of thedecelerating lens allows the ionization energy of the electron beam tobe adjusted without substantially affecting the electron beam generationand deflection.

The gap may be established by one or more ceramic spacers 132 a and 132b, which support each base plate 105 a and 105 b and act as a stand offfrom the source block 35, which is at ionization chamber potential. Theceramic spacers 132 a and 132 b provide both electrical isolation andmechanical support. Note that for clarity, emitter shields 102 and thesource shield 101 are not shown in FIG. 3.

Since the electron entrance apertures 106 a and 106 b can limittransmission of the electron beams, the baseplates 105 a and 105 b canintercept a portion of the energetic electron beams 70 a, 70 b. Thebaseplates 105 a, 105 b must therefore be either actively cooled, orpassively cooled. Active cooling may be accomplished by passing liquidcoolant, such as water, through the baseplates. Alternatively, passivecooling may be accomplished by allowing the baseplates to reach atemperature whereby they cool through radiation to their surroundings.This steady-state temperature depends on the intercepted beam power, thesurface area and emissivity of the baseplates, and the temperatures ofsurrounding components. Allowing the baseplates 105 a, 105 b to operateat elevated temperature, for example at 200C, may be advantageous whenrunning condensable gases which can form contaminating andparticle-forming films on cold surfaces.

FIG. 5C shows a simplified top view of the electron beam-forming regionof the source illustrated in FIGS. 4B and 5B. The filament 110 b is atpotential Ve, for example, −0.5 keV with respect to ionization chamber44 (FIG. 3), and the anode 140 b, the magnetic pole assembly 125 b, baseplate 105 b, and the emitter shield 102 b are all at anode potential Va,for example, 1.5 keV. Thus, the electron beam energy is 2 keV. Theelectron beam 70 b is deflected by magnetic field 135 b in the air gapbetween the poles of the magnetic pole assembly 125 b, such thatelectron beam 70 b passes through the base plate aperture 106 b. Typicalvalues for the base plate apertures 106 a and 106 b and the electronentrance apertures 71 a and 71 b are all 1 cm in diameter, althoughlarger or smaller apertures are possible.

Ionization Probability

FIG. 21 illustrates how ionization probability depends on the electronenergy for electron impact ionization. Ammonia (NH₃) is used as anillustration. Probability is expressed as cross section a, in units of10⁻¹⁶ cm². Electron energy (T) is in eV, i.e., electron-volts. Shown aretwo sets of theoretical curves marked BEB (vertical IP) and BEB(adiabatic IP) calculated from first principles, and two sets ofexperimental data, from Djuric et al. (1981) and from Rao and Srivastava(1992). FIG. 21 illustrates the fact that certain ranges of electronenergies produce more ionization than in other energy ranges. Ingeneral, cross sections are highest for electron impact energies betweenabout 50 eV and 500 eV, peaking at about 100 eV. Thus, the energy withwhich the electron beams enter the ionization. chamber 44 is animportant parameter which affects the operation of the ion source of thepresent invention. The features shown in FIGS. 4A, 4B and FIGS. 5A and5B show how the present invention incorporates electron optics whichallow for broad control of electron impact ionization energy whileoperating at nearly constant conditions in the electron beam-forming anddeflection regions of the ion source.

Temperature Control

One aspect of the ion source of the present invention is user control ofthe ionization chamber temperature, as well as the temperature of thesource block and valves. This feature is advantageous when vaporizingcondensable gases, preventing significant coating of surfaces withcondensed material, and ensuring efficient transport of the vaporthrough conduit 39, valves 100, 110, and vapor feed 32. The sourceutilizes a combination of heating and cooling to achieve accuratecontrol of the source temperature. Separate temperature control isprovided for vaporizer 28, shutoff valves 100 and 110, and source block35. Ionization chamber 44 is passively heated, as is extraction apertureplate 80, by interactions with electron beam 70, and maintains stableoperating temperature though thermally conductive interfaces betweensource block 35 and ionization chamber 44, and between ionizationchamber 44 and extraction aperture plate 80, such that source blocktemp<ionization chamber temp<extraction aperture temp. Externalelectronic controllers (such as an Omron model E5CK) are used fortemperature control. Heating is provided by embedded resistive heaters,whose heating current is controlled by the electronic controller.Cooling is provided by a combination of convective and conductive gascooling methods, as further described, for example, in commonly ownedPCT application US01/18822, and in U.S. application Ser. No. 10/183,768,both herein incorporated by reference.

FIG. 6 illustrates a closed-loop control system for three independenttemperature zones, showing a block diagram of a preferred embodiment inwhich three temperature zones are defined: zone 1 for vaporizer body 30,zone 2 for isolation valves 100 and 110, and zone 3 for the source block35. Each zone may have a dedicated controller; for example, an ConronE5CK Digital Controller. In the simplest case, heating elements aloneare used to actively control temperature above room ambient, forexample, between 18C to 200C or higher. Thus, resistive cartridge-typeheaters can be embedded into the vaporizer body 30 (heater 1) and thesource block 35 (heater 3), while the valves 100, 110 can be wrappedwith silicone strip heaters (heater 2) in which the resistive elementsare wire or foil strips. Three thermocouples labeled TC1, TC2, and TC3in FIG. 6 can be embedded into each of the three components 30, 35,100(110) and continuously read by each of the three dedicated temperaturecontrollers. The temperature controllers 1, 2, and 3 are user-programmedwith a temperature setpoint SP1, SP2, and SP3, respectively. In oneembodiment, the temperature setpoints are such that SP3>SP2>SP1. Forexample, in the case where the vaporizer temperature is desired to be at30C, SP2 might be 50C and SP3 70C. The controllers typically operatesuch that when the TC readback does not match the setpoint, thecontroller's comparator initiates cooling or heating as required. Forexample, in the case where only heating is used to vary temperature, thecomparator output is zero unless TC1<SP1. The controllers may contain alook-up table of output power as a nonlinear function of temperaturedifference SP1-TC1, and feed the appropriate signals to the controller'sheater power supply in order to smoothly regulate temperature to theprogrammed setpoint value. A typical method of varying heater power isby pulse-width modulation of the power supply. This technique can beused to regulate power between 1% and 100% of full scale. Such PIDcontrollers can typically hold temperature setpoint to within 0.2C.

Magnetic Yoke Assembly

In one embodiment, a uniform magnetic field B′ 119 is established withinionization chamber 44 by the incorporation of a permanent magnetic yokeassembly 500, shown in FIG. 7A, into ionization chamber 44. Referringnow to FIG. 7A, magnetic flux is generated by a pair of permanentmagnets, for example, samarium-cobalt magnets 510 a and 510 b, andreturned through yoke assembly 500 through the gap between the C-shapedsymmetrical pole pieces 520 a and 520 b. The electron beam 70 entersthrough the hole 530 a in the yoke 520 a and exits through the hole 530b in the yoke 520 b. FIG. 7C shows how yoke assembly 500 integrates intoionization chamber 44. In FIG. 7B, the ionization chamber 44 has amilled-out section which receives the yoke assembly 500 and the poles520 a and 520 b such that the surface 550 of the yoke assembly 500 andthe surface of the ionization chamber 44 are flush. The interior wall ofthe narrow annulus 540 a and 540 b (not shown), machined as part ofionization chamber 44, defines an electron entrance aperture 70 a and anelectron exit aperture 71, insuring that the ferromagnetic material ofthe yoke assembly 500 is not exposed to the electron beam, reducing anypossibility of ferrous metals contamination within the ionization volumeof ionization chamber 44. FIG. 7C shows lines of flux along across-section containing the xy plane (x is horizontal, y is vertical,antiparallel to the direction of propagation of electron beam 70 asshown in FIG. 5) of yoke assembly 500, calculated with field modelingsoftware. Very uniform field lines 119 are generated within thepropagation volume of electron beam 70. B′ 119 is directed parallel toincoming electron beam 70 in order to confine electron beam 70.

A different embodiment of a magnetic yoke assembly is shown in FIG. 7D.This embodiment consists of a magnetic coil 610, an upper yoke 620 a andan upper pole 630 a, and an lower yoke 630 a and a lower pole 630 b; abobbin core 600 connects the upper yoke 620 a and the lower yoke 630 bin a magnetic circuit which returns flux through the vacuum gap betweenthe upper pole 630 a and the lower pole 630 b. Flux is generated by anelectrical current through the coil 610 wire. The flux is carried bybobbin core 600 to the upper and lower yokes 630 a and 630 b. By varyingthe coil current, the magnetic flux density (i.e., the strength of themagnetic field) can in turn be varied in the vacuum gap.

FIG. 7E shows a cutaway view (containing the Y-Z plane) of the magneticyoke assembly of FIG. 7D integrated into the ion source of the presentinvention. The geometry of the yoke assembly as depicted in FIG. 7Ediffers markedly from the yoke assembly depicted in FIG. 7B. Asignificant departure from FIG. 7B lies in the geometry of the yokes 620a and 620 b, which are oriented along the Y-direction (antiparallel tothe direction of propagation of the ion beam). The yoke assembly of FIG.7E also utilizes a simpler magnetic circuit, having only one pair ofreturn yokes 620 a and 630 b, instead of the two pairs of return yokesin magnetic yoke assembly 500 depicted in FIG. 7A. The coil 610 isembedded in the source block 35 to provide heat sinking of the coil tothe temperature-controlled source block 35 (not shown in FIG. 7E).

FIG. 7F depicts the flux paths and flux density through the magneticyoke assembly of FIG. 7D, the leakage flux is largely restricted to theanterior of the ion source, out of the ion beam path, while a relativelyuniform flux density is produced between the poles 630 a and 630 b,wherein resides the ionization volume containing electron beam 70. Witha coil current of 3000 amp-turns, a magnetic flux density of about 100Gauss can be produced along the Z-direction (a line joining the centerof upper pole 630 a and lower pole 630 b). A user-selectable fluxdensity is thus produced along Z from zero to 100 Gauss by controllingthe electrical current through the coil 610. Referring now to FIG. 7G,flux lines in the X-Z plane, within the ionization region and parallelto the plane containing ion extraction plate 80′ and ion extractionaperture 81′, are shown. The Z-component of flux is quite uniform inthis region directly the ion extraction aperture 81′. The ion extraction81′ aperture would be oriented along Z, in the plane of the paper.

FIG. 7H depicts the incorporation of a high-permeability magnetic shield640 under the baseplate 105 of the electron gun, in order to prevent thefield produced by pole 630 a from penetrating into region 650 whereinthe electron beam is guided through 90 degrees. Without shield 640,stray magnetic fields along the vertical or y-direction would causeunwanted deflection of the electron beam in the lateral or x-direction,causing an error in the trajectory 660 of the electron beam prior toentering ionization chamber 44.

By incorporating the magnetic yoke assembly of FIG. 7B as shown in FIG.7H in the ion source of FIG. 4A, for example, it is realized that theresulting use of a confining magnetic field helps to counteractdispersive space-charge forces which would blow up electron beam 70subsequent to deceleration, i.e., as it enters ionization chamber 44.This has the benefit of enabling higher charge density in electron beam70, hence higher ion density near to the preferred ionization regionadjacent to ion extraction aperture 81, resulting in increased ioncurrent 120. Further gains may be realized by biasing beam dump 72 to anegative voltage Vr, relative to ionization chamber 44, by power supply117. For example, if Vr≦Vc, then a reflex mode may be established,whereby primary electrons contained in electron beam 70 are reflectedfrom beam dump 72, increasing the effective path length of theelectrons. At sufficiently low electron energies, the presence ofconfining field B′ 119 causes the reflected electrons to execute ahelical trajectory along the direction of B′. We note that B 135 and B′119 are orthogonal in direction, B 135 deflecting the electron beam 70into ionization chamber 44 and B′ 119 confining the resultant beam;therefore magnetic shield 118 is added to the bottom of base plate 105.Magnetic shield 118 is made of high-permeability metal so as to preventthe two fields from mixing; this separates the path of electron beam 70into two regions of magnetic field; outside ionization chamber 44, andwithin ionization chamber 44.

Method For Generating Boron Hydride Cluster Ions

The method herein described can be considered normal operation of theion source of the present invention where the only difference from otheroperational modes is the user's choice of values for the sourceparameters (feed material, feed gas flow rate, electron ionizationenergy and current, and source component temperature(s)). Solidoctadecaborane, B₁₈H₂₂ may be used, to produce boron hydride clusterions of the form B₁₈H_(x) ⁺, by using the vaporizer and ion sourcedepicted in FIG. 3. Octadecaborane is a stable solid at room temperatureand has a vapor pressure of a few millitorr. In order to generate usefulmass flows of about 1 sccm of octadecaborane vapor 32, the vaporizer 28may be held at about 90C. FIG. 8A displays a plot of two variables as afunction of vaporizer temperature: vaporizer pressure on the rightvertical axis, and ion current delivered to the post-analysis Faradaycup of a high-current implanter similar to that depicted in FIG. 1 d.Referring back to FIG. 3, the vaporizer pressure was measured by acapacitance manometer 60 in communication with a valve 110. Typicalsource operating parameters were: valve (100 and 110) temperature =120C,source block 35 temperature=120C, electron ionization energy=1 keV,electron beam current≈70 mA. This was achieved by setting Vc=−1 kV,Va=1.3 kV, Vr=−1 kV, and filament emission current=160 mA.

FIG. 8 b illustrates the molecular structure of B,₈H₂₂, and shows therelative positions of H atoms (light spheres) and B atoms (darkspheres).

FIG. 9 shows an octadecaborane mass spectrum collected under similarconditions to those used to generate FIG. 8A, in a cluster ionimplantation system similar to that disclosed in FIG. 1D. The variableresolving aperture 270 was set to a high mass resolution, which selecteda four-AMU wide ion beam 240 to a downstream Faraday cup. FIG. 10 showsan octadecaborane mass spectrum for both negative and positive ions,collected under conditions similar to those used to generate the data ofFIG. 9. The polarity of all the implanter power supplies were reversedto switch between negative and positive ions, which were collectedwithin a few minutes of one another and recorded on the same plot. TheB₁₈H_(x) ⁺ and B₁₈H_(x) ⁻ peaks are at 210 AMU, suggesting a mostprobable chemical formula for the ions of B₁₈H₁₆ ⁺ and B₁₈H₁₆ ⁻,respectively. FIG. 11A was collected under conditions similar to thoseused to collect the data of FIG. 9, but with the resolving aperture 270set to allow about 18 AMU to pass downstream, allowing much higherB₁₈H_(x) ⁺ currents. However, the lack of structure in the main peakattests to reduced mass resolution. FIG. 11B is a detail collected athighest mass resolution. With the resolving aperture set at<1 mm, only asingle AMU was passed downstream to the Faraday. Thus, individual boronhydride peaks separated by one AMU are clearly visible. FIG. 12 shows aplot of beam current at the Faraday versus extraction voltage withoutany deceleration of the ion beam, collected at the low mass resolutionof FIG. 11A. FIG. 13 shows the data of FIG. 12 converted to atomic boroncurrent versus effective implant energy, as a means of comparison withmonomer boron implantation. Atomic boron current=18 X octadecaboraneFaraday current, and effective implant energy=11/210 X extractionvoltage. These currents are many times greater than currently attainablewith conventional monomer boron implantation, particularly without iondeceleration.

In order to characterize the implantation profile of B₁₈H_(x) ⁺ forboron doping of semiconductors, a commercial silicon wafer was dipped inHF solution to remove any native oxide, and implanted in a cluster ionimplantation system similar to that disclosed in FIG. 1D. A boron doseof 2×10¹⁶ cm⁻² was delivered by implanting a B₁₈H_(x) ⁺ dose of 1.1×10¹⁵cm⁻². The B₁₈H_(x) ⁺ ion energy was 20 keV during the implant, resultingin an effective boron implant energy of about 1 keV per boron atom. FIG.20 shows the as-implanted boron profile as determined by SIMS (SecondaryIon Mass Spectrometry). The peak of the profile is at about 50 Å, whichagrees well with a projected range of 58 Å predicted by TRIMcalculations for a 1 keV boron implant.

Formation Of N- And P- Type Shallow Junctions

An important application of this method is the use of cluster ionimplantation for the formation of N- and P-type shallow junctions aspart of a CMOS fabrication sequence. CMOS is the dominant digitalintegrated circuit technology in current use and its name denotes theformation of both N-channel and P-channel MOS transistors (ComplementaryMOS: both N and P) on the same chip. The success of CMOS is that circuitdesigners can make use of the complementary nature of the oppositetransistors to create a better circuit, specifically one that draws lessactive power than alternative technologies. It is noted that the N and Pterminology is based on Negative and Positive (N-type semiconductor hasnegative majority carriers, and vice versa), and the N-channel andP-channel transistors are duplicates of each other with the type(polarity) of each region reversed. The fabrication of both types oftransistors on the same substrate requires sequentially implanting anN-type impurity and then a P-type impurity, while protecting the othertype of devices with a shielding layer of photoresist. It is noted thateach transistor type requires regions of both polarities to operatecorrectly, but the implants which form the shallow junctions are of thesame type as the transistor: N-type shallow implants into N-channeltransistors and P-type shallow implants into P-channel transistors. Anexample of this process is shown in FIGS. 14 and 15. In particular, FIG.14 illustrates a method for forming the N-channel drain extension 89through an N-type cluster implant 88, while FIG. 15 shows the formationof the P-channel drain extension 90 by a P-type cluster implant 91 It isto be noted that both N- and P-types of transistors Requires shallowjunctions of similar geometries, and thus having both N-type and P-typecluster implants is advantageous for the formation of advanced CMOSstructures.

An example of the application of this method is shown in FIG. 16 for thecase of forming an NMOS transistor. This figure shows semiconductorsubstrate 41 which has undergone some of the front-end process steps ofmanufacturing a semiconductor device. For example, the structureconsists of a N-type semiconductor substrate 41 processed through theP-well 43, trench isolation 42, and gate stack formation 44, 45 steps.An exemplary process for forming the gate stack, P-well and trenchisolation is disclosed in co-pending patent application PCT/US03/19085,filed on Jun. 18, 2003, entitled “A semiconductor Device and Method ofFabricating a Semiconductor Device”, attorney docket no. 211843/00030.

The P-well 43 forms a junction with the N-type substrate 41 thatprovides junction isolation for the transistors in the well 43. Thetrench isolation 42 provides lateral dielectric isolation between the N-and P- wells (i.e., in the overall CMOS structure). The gate stack isconstructed, with a gate oxide layer 44 and a polysilicon gate electrode45, patterned to form a transistor gate stack. A photoresist 46 isapplied and patterned such that the area for NMOS transistors isexposed, but other areas of the substrate 41 are shielded. After thephotoresist 46 is applied, the substrate 41 is ready for the drainextension implant, which is the shallowest doping layer required by thedevice fabrication process. A typical process requirement forleading-edge devices of the 0.13 μm technology node is an arsenicimplant energy of between 1 keV and 2 keV, and an arsenic dose of 5×10¹⁴cm⁻². The cluster ion beam 47, As₄H_(x) ⁺ in this case, is directed atthe semiconductor substrate, typically such that the direction ofpropagation of the ion beam is normal to the substrate, to avoidshadowing by the gate stack. The energy of the AS₄H_(x) ⁺ cluster shouldbe four times the desired As+implant energy, e.g., between 4 keV and 8keV. The clusters dissociate upon impact with the substrate, and thedopant atoms come to iest in a shallow layer near the surface of thesemiconductor substrate, which forms the drain extension region 48. Wenote that the same implant enters the surface layer of the gateelectrode 49, providing additional doping for the gate electrode. Theprocess described in FIG. 16 is thus one important application of theproposed invention.

A further example of the application of this method is shown in FIG. 17:the formation of the deep source/drain regions. This figure shows thesemiconductor substrate 41 of FIG. 16 after execution of furtherprocesses steps in the fabrication of a semiconductor device. Theadditional process steps include the formation of a pad oxide 51 and theformation of spacers 52 on the sidewalls of the gate stack. The padoxide 51 is a thin layer of oxide (silicon dioxide) used to protect theexposed substrate areas, the top of the gate electrode 49 and thepotentially exposed gate dielectric edge. The pad oxide 51 is typicallythermally grown to a thickness of 5-10 nm. The spacer 52, on the otherhand, is a region of dielectric, either silicon dioxide, siliconnitride, or a combination of these, which resides on the side of thegate stack and serves to insulate the gate electrode. It also serves asan alignment guide for the source/drain implant (e.g., 54), which mustbe spaced back from the gate edge for the transistor to operateproperly. The spacers 52 are formed by the deposition of silicon dioxideand/or silicon nitride layers which are then plasma etched in a way toleave a residual layer on the side of the gate stack while clearing thedielectrics from the source/drain region.

After etching the spacers 52, a photoresist layer 53 is applied andpatterned to expose the transistor to be implanted, an NMOS transistorin this example. Next, the ion implant to form the source and drainregions 55 is performed. Since this implant requires a high dose at lowenergy, it is an appropriate application of the proposed clusterimplantation method. Typical implant parameters for the 0.13 μmtechnology node are approximately 6 keV per arsenic atom (54) at anarsenic dose of 5×10¹⁵ cm⁻², so it requires a 24 keV, 1.25×10¹⁵ cm⁻²As₄H_(x) ⁺ implant, a 12 keV, 2.5×10¹⁵ cm⁻² As₂H_(x) ⁺ implant, or a 6keV, 5×10¹⁵ cm⁻² As⁺ implant. As shown in FIG. 16, the source and drainregions 55 are formed by this implant. These regions provide a highconductivity connection between the circuit interconnects (to be formedlater in the process) and the intrinsic transistor defined by the drainextension 48 in conjunction with the channel region 56 and the gatestack 44, 45. The gate electrode 45 can be exposed to this implant (asshown), and if so, the source/drain implant provides the primary dopingsource for the gate electrode. This is shown in FIG. 17 as the polydoping layer 57.

The detailed diagrams showing the formation of the PMOS drain extension148 and PMOS source and drain regions 155 are shown in FIGS. 18 and 19,respectively. The structures and processes are the same as in FIGS. 17and 18 with the dopant types reversed. In FIG. 18, the PMOS drainextension 148 is formed by the implantation of a boron cluster implant147. Typical parameters for this implant would be an implant energy of500 eV per boron atom with a dose of 5×10¹⁴ cm⁻², for the 0.13 μmtechnology node. Thus, a B₁₈H_(x) ⁺ implant at 211 AMU would be at 9.6keV at an octadecaborane dose of 2.8×10¹³ cm⁻². FIG. 19 shows theformation of the PMOS source and drain regions 148, again by theimplantation of a P-type cluster ion beam 154 such as octadecaborane.Typical parameters for this implant would be an energy of around 2 keVper boron atom with a boron dose of 5×10¹⁵ cm⁻² (i.e., 38.4 keVoctadecaborane at 2.8×10¹⁴ cm⁻²) for the 0.13 μm technology node.

In general, ion implantation alone is not sufficient for the formationof an effective semiconductor junction: a heat treatment is necessary toelectrically activate the implanted dopants. After implantation, thesemiconductor substrate's crystal structure is heavily damaged(substrate atoms are moved out of crystal lattice positions), and theimplanted dopants are only weakly bound to the substrate atoms, so thatthe implanted layer has poor electrical properties. A heat treatment, oranneal, at high temperature (greater than 900C) is typically performedto repair the semiconductor crystal structure, and to position thedopant atoms substitutionally, i.e., in the position of one of thesubstrate atoms in the crystal structure. This substitution allows thedopant to bond with the substrate atoms and become electrically active;that is, to change the conductivity of the semiconductor layer. Thisheat treatment works against the formation of shallow junctions,however, because diffusion of the implanted dopant occurs during theheat treatment. Boron diffusion during heat treatment, in fact, is thelimiting factor in achieving USJ's in the sub-0.1 micron regime.Advanced processes have been developed for this heat treatment tominimize the diffusion of the shallow implanted dopants, such as the“spike anneal”. The spike anneal is a rapid thermal process wherein theresidence time at the highest temperature approaches zero: thetemperature ramps up and down as fast as possible. In this way, the hightemperatures necessary to activate the implanted dopant are reachedwhile the diffusion of the implanted dopants is minimized. It isanticipated that such advanced heat treatments would be utilized inconjunction with the present invention to maximize its benefits in thefabrication of the completed semiconductor device.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

1. A method of implanting ions comprising the steps of: (a) producing avolume of gas phase molecules of a boron hydride B_(n)H_(m), where n andm are integers and n>10 and m≧0; (b) ionizing the boron hydridemolecules defining ionized boron hydride molecules; and (c) acceleratingthe ionized boron hydride molecules by an electric field into a target.2-39. (canceled)